1. Field of the Invention
The present invention relates to tube-to-header joints in heat exchanger core constructions and, more particularly, to an improved diamond-shaped tube-to-header joint configuration which offers improved manufacturability and durability and is particularly suited for, but not necessarily limited to, use in heavy duty heat exchanger applications such as engine cooling radiator assemblies. The present invention resides in providing a heat exchanger core construction wherein each of the respective end portions of conventional heat exchanger tube members are formed into a diamond-shaped configuration and mating header plates are constructed to include corresponding diamond-shaped openings for receiving the respective tube end portions therethrough.
2. Description of the Prior Art
The tube-to-header joint interface can be one of the most labor intensive aspects of conventional heat exchanger core assembly manufacture. This is due primarily to the number of tubes and hence joints included in a typical heat exchanger construction. Each end of a conventional tube member must be aligned and then inserted into its respective header slot. While alignment is somewhat predetermined by the engagement of the tubes and the heat transfer fins in plate-fin cores or by assembly fixturing in the case of serpentine fin cores, manufacturing tolerances generally require additional operations to ensure that the proper pre-soldering fit between the tubes and the header slots is obtained. For example, in core assemblies utilizing plate-type fin elements, the tubes are inserted through a plurality of adjacent stacked plate members and this somewhat predetermines the alignment of the tube ends in relation to the respective header slots. In core assemblies utilizing serpentine-type fin elements, the tubes are assembly fixtured between adjacent serpentine fin elements and this fixturing also somewhat predetermines the alignment of the tube ends in relation to the respective header slots. Even though alignment is, in general, predetermined in both cases, additional operations such as tube end opening/flaring operations are still required to ensure that a proper pre-bonding fit between the tube end portions and the header slots is obtained. Opening/flaring operations help to reduce the clearance between the tube end portions and the tube slots in the header plates and this helps to provide a leakproof fit therebetween. Such opening/flaring steps are not optimal because they exert a considerable amount of stress on the tube walls in the general area of the tube end portion thereby reducing the strength of the tube walls.
Once the tube members are aligned and inserted into the respective header slots, a solder joint or other type of bonding must in turn be effected at each tube-to-header interface. These joints are typically effected by the application of a solder coating to one side of the header plate. The coating is pulled by capillary action into the interface between the outside perimeter of the tube and the tube slot flanges associated with each of the header plates. This solder coating may also be applied by dipping the face of the header plate into a molten solder bath. Alternatively, the solder coating may be applied by spraying molten solder onto the air side (fin side) of the header plate while simultaneously applying heat to the opposite side (coolant side) thereof. These solder joints must be leak free and must be able to endure the load acting thereon between the tubes and header plates resulting from pressure variations, temperature variations, and mechanical stresses occurring during subsequent manufacturing steps as well as during end use operations.
Solder joint strength is generally dependent upon the pre-soldering fit between mating parts. In the case of conventional heat exchanger core constructions, the pre-soldering or pre-bonding fit is determined by the degree of precision in performing the tube end opening/flaring steps to reduce the clearance between the tube end portions and the tube slots in the header plates. The poor fatigue characteristics of soft solder combined with the poor pre-soldering fit in conventional heat exchanger core assemblies and the reduced strength of the tube walls in the general area of the tube end which is caused during the opening/flaring steps results in a tube-to-header interface which is typically the weakest area in a conventional heat exchanger core assembly and hence in a conventional radiator assembly. Thus, since the overall structural integrity of a heat exchanger core assembly is dependent upon the tube-to-header interface, improving the structural integrity of the tube-to-header interface will greatly improve the overall structural integrity of the heat exchanger core assembly and, consequently, the resultant radiator assembly.
In addition to the structural integrity problems associated with conventional heat exchanger core assemblies, there are also efficiency problems associated therewith. For example, associated with the various optional tube-to-header soldering operations described above is the propensity of solder to bridge the gap between opposing side portions of the tubes and either partially or completely obstruct coolant flow therethrough. This can be a distinct problem in core constructions utilizing high aspect ratio (narrow width) tubing which is much more susceptible to tube end plugging during the soldering or brazing process. Tube end plugging that occurs when solder bridges the gap between opposing sides of the tube end portions is undesirable because such plugging has a disparaging effect on heat transfer performance and increases the pumping power required for coolant flow circulation. Plugging may also occur in the tube end portions as a result of blooming corrosion which occurs during the service life of the assembly. Blooming corrosion is generally concentrated at the tube-to-header joint interface and can sometimes bridge the narrow gap between the sides of conventional heat exchanger tube members. Tube end plugging caused by blooming corrosion is likewise undesirable since it also has a disparaging effect on the overall heat transfer performance of the unit and also increases the pumping power required for coolant flow circulation. Thus, reducing the amount of tube end plugging that occurs as a result of either solder bridging or blooming corrosion will greatly increase the overall efficiency of heat exchanger core assemblies.
Other problems associated with conventional heat exchanger assemblies include collapsing of the side walls and/or end portions of the tube members extending therethrough generally known as "hour glassing" which often occurs during the various stages of the manufacturing process. This is particularly true of the oval-type tube members and may occur during the initial installation of the header plates if there is a mismatch in either cross-sectional height or width between the tube ends and the header slots. Collapsing may also occur during the tube-to-header soldering operation. In this case, the rapid heating of the relatively thin tube walls causes the tube cross-sectional perimeter to grow more rapidly than the header slot inside perimeter and produces a similar interference problem between the tube and header slot. Elimination of these problems will greatly increase the production of and will significantly reduce manufacturing time for heat exchanger core assemblies.
Many attempts have been made to solve the problems associated with conventional heat exchanger core assemblies including attempts to increase both the efficiency of heat exchanger core assemblies and improve the tube-to-header joint interface thereby improving the overall structural integrity of the heat exchanger core construction. For example, Melnyk U.S. Pat. No. 4,458,749 discloses a tube-to-header arrangement including a tube member having an oblong or flattened body which is reformed at its respective end portions into a cylindrical configuration for insertion into mating header plates; Moranne U.S. Pat. No. 4,369,837 discloses a tube-to-header joint interface wherein each respective tube end portion is formed into a figure 8 configuration for insertion into mating header plates; and Donaldson U.S. Pat. No. 3,497,936 discloses a method of making a heat exchanger wherein conventional flat-oval tubes are reformed at their respective tube end portions into a rectangular shape for insertion into mating header plates. In addition, Lesniak U.S. Pat. No. 4,513,811, Baldensperger et al U.S. Pat. No. 4,465,129, and Melnyk U.S. Pat. No. 4,234,041 all disclose heat exchanger constructions utilizing header plates having tube slots incorporated therein with an increased flange area extending at least partially therearound for increasing the solder joint surface between the tube slots and the tubes extending therethrough. All such prior art constructions have proved to be less than desirable in both sufficiently reducing tube end plugging and improving the tube-to-header joint interface in an effort to improve the overall structural integrity and efficiency of the resultant heat exchanger assemblies.
The present invention overcomes many of the disadvantages and shortcomings associated with known heat exchanger core constructions, including the above-identified prior art constructions, and improves the structural integrity of the tube-to-header joint by improving the pre-soldering or pre-bonding fit between tube member end portions and the respective openings in the mating header plates by controlling the joint clearances, by providing a mechanically locked joint design and by increasing the distance between the opposing side portions of the individual tube ends thereby reducing or eliminating tube end plugging. The present tube-to-header joint design also reduces or eliminates the need for performing opening/flaring operations on each tube end subsequent to header installation and offers the potential for significantly increased service life for the entire heat exchanger assembly.